Conventional AC adapters have limited output current. As such, such conventional AC adapters may not be able to support fast battery charging when a system load powered by a battery is ON (in operation). For example, one of these conventional charging methods uses an AC adapter which can either power the system or charge the battery. As such, the battery cannot be charged when the system is ON. Another method is to use an AC adapter which has the capability to power the system and also charge the battery at a high charging current simultaneously. However, this kind of AC adapter is complex and expensive.
A third method is to use an AC adapter which charges the battery at a constant high charging current when the system is OFF, and charges the battery at a constant low charging current when the system is ON. However, when the charging current is changed from high to low, a false charge termination may occur since a negative slope of the battery voltage may exceed a predetermined threshold. For example, for metal-hydride and nickel-cadmium batteries, the charging process is terminated when a negative slope of the battery voltage exceeds a certain threshold. In addition, a charging time determined by such a method is not accurate due to a variation of the charging current.